A conventional TMR sensor includes a seed layer, an anti-ferromagnetic (AFM) layer having a blocking temperature Tb, synthetic anti-parallel (SyAP) layers, one of which serves as the reference layer, a barrier layer, a free layer, and one or more capping layers. As TMR sensors become smaller, sensor noise improvement, especially at high temperatures, is critical for maintaining or improving the signal to noise ratio (SNR) and the bit error ratio (BER) (the number of bit errors divided by the total number of transferred bits).
A Ta based seed layer, such as Ta/Ru, Ta/Cr, Ta/Cu or Ta/NiCr, is usually used as the seed layer for a TMR sensor but it is very difficult to improve the thermal stability of such seeds without increasing AFM and/or seed thickness. This leads to an increase in shield-to-shield spacing, resulting in a resolution penalty.
FIG. 1 shows a TMR sensor of the prior art. Depicted there are bottom and top shields, 11 and 21 respectively, conventional seed layer 13 (ranging in thickness from 5 A to 30 A), AFM layer 14 (ranging in thickness from 50 A to 100 A), antiparallel (AP)2 layer 15, AP coupling layer 16 (usually of ruthenium), and AP1 (ranging from 10 A to 40 A), the latter also serving as the reference layer. AP1 is followed by insulated tunnel barrier layer 18 on which lies free layer 19 followed by capping layer(s) 20.